The invention relates to a hydraulic brake booster for a dual circuit braking system such as that described in the German Pat. No. 2,312,641. In brake boosters of this type, the problem arises that if the reservoir pressure fails, the travel spring excursion represents an unnecessary waste motion which thus lengthens the pedal travel of the brake booster upon the failure of the auxiliary energy source.
The U.S. Pat. No. 4,094,554 discloses a solution for this problem by provision of a shut-off mechanism for the travel spring excursion which is dependent upon supply pressure. However, a shut-off mechanism of the above-noted type provides a consequent disadvantage that, because supply pressure failure seldom occurs, the mechanism is actuated only at rare and infrequent intervals of time. As a result of this inactivity, the mechanism can become sluggish and may not be in operable condition when needed, as in case of emergency.
In integrated hydraulic brake boosters having a control valve located in parallel with the booster cylinder or cylinders, the problem is still more complex. The push rod which actuates the control rod must come into contact with a stop after it has switched the control valve to the braking direction in order to provide the driver with braking "feel", i.e., with a sense of the braking force which has been brought to bear. This is achieved via the travel spring excursion disposed between the control valve and the brake pedal. Should this stop not be present upon attaining the point where the control valve becomes ineffective, then the pedal force would increase no further; instead, the pedal force would remain constant until the pedal push rod finally reached the main cylinder piston. Only then, upon a continuing depression of the pedal, would a further increase in the pedal return force or brake feel occur. Such a characteristic in a vehicular braking system is considered unfavorable and undesirable from the standpoint of human engineering.
To alleviate this braking characteristic, it is known in the art to use an auxiliary piston dependent upon supply pressure; thus, if the supply pressure fails, this auxiliary piston pushes open a check valve which previously had retained closed a chamber, the movable wall of which chamber included a support piston for the control valve. Such a mechanism also suffers from the disadvantage that it is actuated very seldom, so that it may become sluggish or inoperable. Thus, in the event of a failure, a dangerous defect in the shut-off mechanism which would hinder optimal functioning of the brake booster could remain undetected.